The invention is based on a hydraulic anti-lock brake system especially for motor vehicles as set forth hereinafter. German Patent Application 34 19 311 A1 has already disclosed a hydraulic brake system with a pressure control device, provided in the rear axle brake circuit, that is switchable by impingement with a control pressure and that in the event the anti-skid system fails acts as a proportional pressure regulating valve and reduces the brake pressure in the rear wheel brake cylinders by a fixed ratio to the master brake cylinder pressure, so that locking of the rear wheels is prevented prior to locking of the front wheels of the vehicle. The known pressure control valve has the disadvantage of a very complicated and expensive design, with many components and a large structural size. The production cost of the pressure control device is also high. The control pressure generated by an auxiliary pressure source is switched by an electromagnetic control valve, which is very complicated and expensive. Moreover, it is possible that in the event of a malfunction of the control valve or auxiliary pressure source, even if the anti-skid system is intact, the brake pressure in the rear wheel brake cylinders may undesirably be decreased, since if the control pressure is absent, the pressure control device always acts as a proportional pressure regulating valve.